The present invention generally relates to a constant current source circuit and, more particularly, to a constant current source circuit suitable for battery-based applications.
Recently, an electronic circuit has been demanded which can operate over a wide power source voltage range. In some applications, typically, battery-based applications, an electronic circuit designed to operate with a 5 V-based standard power source voltage is required to stably operate with a decreased power source voltage of 3 volts or 2 volts, for example. The present invention is directed to a constant current source circuit capable of providing an electronic circuit with sufficient current even when the power source voltage decreases so that the electronic circuit can operate correctly.
Referring to FIG. 1A, there is illustrated a conventional constant current source circuit (see T. Saito et al., "DTMF/PULSE DIALER LSI", The Institute of Electronics and Communication Engineers of Japan Integrated Nationalwide Meetings, pp. 2-176, 1985, for example). The illustrated circuit includes an npn-type bipolar transistor (hereinafter simply referred to as a transistor) 1. A load resistor 7 is connected to the emitter of the transistor 1, and a resistor 2 is connected between the base and the emitter. A current Iref passes through the resistor 2. A current mirror circuit 4 utilizes the current Iref as a reference current, and supplies a load circuit 5 with an output current Io. As shown in FIG. 1B, the current mirror circuit 4 is made up of two p-channel MOS transistors 4a and 4b.
A current Ia passing through the resistor 7 is written: EQU Ia=Ic+Iref=(1+.beta.)Iref (1)
where Ic is the collector current, and B is the current transfer ratio of the transistor I. The current Ia is written as follows also: EQU Ia=Va/r.sub.1 ( 2)
where Va is a voltage across the resistor 7, and r.sub.1 is a resistance of the resistor 7. The voltage Va is equal to a voltage obtained by subtracting the sum of a voltage drop caused in the current mirror circuit 4 and a base-emitter voltage V.sub.BE of the transistor 1 from a positive power source voltage V.sub.DD. That is, the voltage Va across the resistor 7 is expressed as follows: EQU Va=V.sub.DD -[(.vertline.V.sub.th .vertline.-.DELTA..sub.1) +(V.sub.BE +.DELTA..sub.2)] (3)
where .vertline.V.sub.th .vertline. is an absolute value of the threshold voltage of the MOS transistor 4a, .DELTA..sub.1 is an error voltage of the voltage V.sub.th, and .DELTA..sub.2 is an error voltage of the base-emitter voltage V.sub.BE.
Normally, the sum of the absolute value of the threshold voltage V.sub.th and the error voltage .DELTA..sub.1 is approximately 1.0 V, and the sum of the base-emitter voltage V.sub.BE and the error voltage .DELTA..sub.2 is approximately 0.7 V. In this case, when the power source voltage V.sub.DD is equal to 5 V, the voltage Va (hereinafter referred to as Va.sub.1 with equal to 5 V) is approximately 3.3 V. In this case, the current Ia (Ia.sub.1) is EQU Ia.sub.1 =3.3/r.sub.1. (4)
When the power source voltage V.sub.DD is equal to 2 V, the voltage Va (hereinafter referred to as Va.sub.2 with V.sub.DD equal to 2 V) is approximately 0.3 V. In this case, the current Ia (Ia.sub.2) is as follows: EQU Ia.sub.2 =0.3/r.sub.1. (5)
The following formula can be obtained from the formulas (4) and (5): EQU Ia.sub.2 =I.sub.a1 /11. (6)
That is, the current Ia.sub.2 with equal to 2 V is one-eleventh as large as the current Ia.sub.1 with equal to 5 V. Thus, the output current Io decreases drastically, which causes a malfunction of the load circuit 5. For example, load circuit 5 may oscillate, or the frequency characteristics thereof may change.